Communication network having a plurality of bridging nodes which transmit a beacon to terminal nodes in power saving state that it has messages awaiting delivery

ABSTRACT

An apparatus and a method for routing data in a radio data communication system having one or more host computers, one or more intermediate base stations, and one or more RF terminals organizes the intermediate base stations into an optimal spanning-tree network to control the routing of data to and from the RF terminals and the host computer efficiently and dynamically. Communication between the host computer and the RF terminals is achieved by using the network of intermediate base stations to transmit the data.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a continuation in part of application Ser.No. 08/545,108 filed Oct. 19, 1995, now U.S. Pat. No. 5,940,771 issuedAug. 17, 1999, which itself is a continuation of U.S. application Ser.No. 07/947,102 filed Sep. 14, 1992, now abandoned. Said application Ser.No. 07/947,102 is a continuation in part of U.S. application Ser. No.07/907,927 filed Jun. 30, 1992, now abandoned, which is a continuationin part of: 1) U.S. application Ser. No. 07/857,603 filed Mar. 30, 1992,now abandoned; 2) PCT Application No. PCT/US92/03982 filed May 13, 1992,now abandoned; and 3) U.S. application Ser. No. 07/802,348 filed Dec. 4,1991, now abandoned, which is itself a continuation in part of U.S.application Ser. No. 07/790,946 filed Nov. 12, 1991, now abandoned.

BACKGROUND OF TEE INVENTION

In a typical radio data communication system having one or more hostcomputers and multiple RF terminals, communication between a hostcomputer and an RF terminal is provided by one or more base stations.Depending upon the application and the operating conditions, a largenumber of these base stations may be required to adequately serve thesystem. For example, a radio data communication system installed in alarge factory may require dozens of base stations in order to cover theentire factory floor.

In earlier RF (Radio Frequency) data communication systems, the basestations were typically connected directly to a host computer throughmulti-dropped connections to an Ethernet communication line. Tocommunicate between an RF terminal and a host computer, in such asystem, the RF terminal sends data to a base station and the basestation passes the data directly to the host computer. Communicatingwith a host computer through a base station in this manner is commonlyknown as hopping. These earlier RF data communication systems used asingle-hop method of communication.

In order to cover a larger area with an RF data communication system andto take advantage of the deregulation of the spread-spectrum radiofrequencies, later-developed RF data communication systems are organizedinto layers of base stations. As in earlier RF data communicationssystems, a typical system includes multiple base stations whichcommunicate directly with the RF terminals and the host computer. Inaddition, the system also includes intermediate stations thatcommunicate with the RF terminals, the multiple base stations, and otherintermediate stations. In such a system, communication from an RFterminal to a host computer may be achieved, for example, by having theRF terminal send data to an intermediate station, the intermediatestation send the data to a base station, and the base station send thedata directly to the host computer. Communicating with a host computerthrough more than one station is commonly known as a multiple-hopcommunication system.

Difficulties often arise in maintaining the integrity of suchmultiple-hop RF data communication systems. The system must be able tohandle both wireless and hard-wired station connections, efficientdynamic routing of data information, RF terminal mobility, andinterference from many different sources.

SUMMARY OF THE INVENTION

The present invention solves many of the problems inherent in amultiple-hop data communication system. The present invention comprisesan RF Local-Area Network capable of efficient and dynamic handling ofdata by routing communications between the RF Terminals and the hostcomputer through a network of intermediate base stations.

In one embodiment of the present invention, the RF data communicationsystem contains one or more host computers and multiple gateways,bridges, and RF terminals. Gateways are used to pass messages to andfrom a host computer and the RF Network. A host port is used to providea link between the gateway and the host computer. In addition, gatewaysmay include bridging functions and may pass information from one RFterminal to another. Bridges are intermediate relay nodes which repeatdata messages. Bridges can repeat data to and from bridges, gateways andRF terminals and are used to extend the range of the gateways.

The RF terminals are attached logically to the host computer and use anetwork formed by a gateway and the bridges to communicate with the hostcomputer. To set up the network, an optimal configuration for conductingnetwork communication spanning tree is created to control the flow ofdata communication. To aid understanding by providing a more visualdescription, this configuration is referred to hereafter as a “spanningtree” or “optimal spanning tree”.

Specifically, root of the spanning tree are the gateways; the branchesare the bridges; and non-bridging stations, such as RF terminals, arethe leaves of the tree. Data are sent along the branches of the newlycreated optimal spanning tree. Nodes in the network use a backwardlearning technique to route packets along the correct branches.

One object of the present invention is to route data efficiently,dynamically, and without looping. Another object of the presentinvention is to make the routing of the data transparent to the RFterminals. The RF terminals, transmitting data intended for the hostcomputer, are unaffected by the means ultimately used by the RF Networkto deliver their data.

It is a further object of the present invention for the network to becapable of handling RF terminal mobility and lost nodes with minimalimpact on the entire RF data communication system.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a functional block diagram of an RF data communication systemincorporating the RF local-area network of the present invention.

FIG. 2 is a flow diagram illustrating a bridging node's construction andmaintenance of the spanning tree.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a functional block diagram of an RF data communication system.In one embodiment of the present invention, the RF data communicationsystem has a host computer 10, a network controller 14 and bridges 22and 24 attached to a data communication link 16. Also attached to thedata communication link 16 is a gateway 20 which acts as the root nodefor the spanning tree of the RF data network of the present invention. Abridge 42 is attached to the gateway 20 through a hard-wiredcommunication link and bridges 40 and 44 are logically attached togateway 20 by two independent RF links. Additional bridges 46, 48, 50and 52 are also connected to the RF Network and are shown in the FIG. 1.Note that, although shown separate from the host computer 10, thegateway 20 (the spanning tree root node) may be part of host computer10.

The FIG. 1 further shows RF terminals 100 and 102 attached to bridge 22via RF links and RF terminal 104 attached to bridge 24 via an RF link.Also, RF terminals 106, 108, 110, 112, 114, 116, 118, and 120 can beseen logically attached to the RF Network through their respective RFlinks. The RF terminals in FIG. 1 are representative of non-bridgingstations. In alternate embodiments of the present invention, the RFNetwork could contain any type of device capable of supporting thefunctions needed to communicate in the RF Network such as hard-wiredterminals, remote printers, stationary bar code scanners, or the like.The RP data communication system, as shown in FIG. 1, represents theconfiguration of the system at a discrete moment in time after theinitialization of the system. The RF links, as shown, are dynamic andsubject to change. For example, changes in the structure of the RF datacommunication system can be caused by movement of the RF terminals andby interference that affects the RF communication links.

In the preferred embodiment, the host computer 10 is an IBM 3090, thenetwork controller 14 is a model RC3250 of the Norand Corporation, thedata communication link 16 is an Ethernet link, the nodes 20, 22, 24,40, 42, 44, 46, 48, 50 and 52 are intelligent base transceiver units ofthe type RB4000 of the Norand Corporation, and the RF terminals 100,102, 104, 106, 108, 110, 112, 114, 116, 118 and 120 are of type RT1100of the Norand Corporation.

The optimal spanning tree, which provides the data pathways throughoutthe communication system, is stored and maintained by the network as awhole. Each node in the network stores and modifies information whichspecifies how local communication traffic should flow. Optimal spanningtrees assure efficient, adaptive (dynamic) routing of informationwithout looping.

To initialize the RF data communication system, the gateway 20 and theother nodes are organized into an optimal spanning tree rooted at thegateway 20. To form the optimal spanning tree, in the preferredembodiment the gateway 20 is assigned a status of ATTACHED and all otherbridges are assigned the status UNATTACHED. The gateway 20 is consideredattached to the spanning tree because it is the root node. Initially,all other bridges are unattached and lack a parent in the spanning tree.At this point, the attached gateway node 20 periodically broadcasts aspecific type of polling packet referred to hereafter as “HELLOpackets”. The HELLO packets can be broadcast using known methods ofcommunicating via radio frequency (RF) link or via a direct wire link.In the preferred embodiment of the present invention, the RF link iscomprised of spread-spectrum transmissions using a polling protocol.Although a polling protocol is preferred, a carrier-sensemultiple-access (CSMA), busy-tone, or any other protocol might alsomanage the communication traffic on the RF link.

HELLO packets contain 1) the address of the sender, 2) the hoppingdistance that the sender is from the root, 3) a source address, 4) acount of nodes in the subtree which flow through that bridge, and 5) alist of system parameters. Each node in the network is assigned a uniquenetwork service address and a node-type identifier to distinguishbetween different nodes and different node types. The distance of a nodefrom the root node is measured in hops times the bandwidth of each hop.The gateway root is considered to be zero hops away from itself.

FIG. 2 is a flow diagram illustrating a bridge's participation in theconstruction and maintenance of the spanning tree. At a block 201, thebridge begins the local construction of the spanning tree upon power-up.Next, at a block 203, the bridge enters the UNATTACHED state, listeningfor HELLO packets (also referred to as HELLO messages herein) that arebroadcast.

By listening to the HELLO messages, bridges can learn which nodes areattached to the spanning tree. At a block 205, the bridge responds to aHELLO packet received by sending an ATTACH.request packet to the devicethat sent the received HELLO packet. The ATTACH.request packet isthereafter forwarded towards and to the root node which responds bysending an ATTACH.response packet back down towards and to the bridge.

The bridge awaits the ATTACH.response packet at a block 207. Uponreceipt of the ATTACH.response packet, at a block 209, the bridge entersan ATTACHED state. Thereafter, at a block 211, the bridge beginsperiodically broadcasting HELLO packets and begins forwarding orrelaying packets received. Specifically, between HELLO packetbroadcasts, the bridge listens for HELLO, DATA, ATTACH.request andATTACH.response packets broadcast by other devices in the communicationnetwork. Upon receiving such a packet, the bridge branches to a block213. At the block 213, if the bridge detects that it has become detachedfrom the spanning tree the bridge will branch back to the block 203 toestablish attachment. Note that although the illustration in FIG. 2places block 213 immediately after the block 211, the bridgesfunctionality illustrated in block 213 is actually distributedthroughout the flow diagram.

If at the block 213 detachment has not occurred, at a block 214, thebridge determines if the received packet is a HELLO packet. If so, thebridge analyzes the contents of the HELLO packet at a block 215 todetermine whether to change its attachment point in the spanning tree.In a preferred embodiment, the bridge attempts to maintain attachment tothe spanning tree at the node that is logically closest to the rootnode.

The logical distance, in a preferred embodiment, is based upon thenumber of hops needed to reach the root node and the bandwidth of thosehops. The distance the attached node is away from the root node is foundin the second field of the HELLO message that is broadcast. In anotherembodiment of the present invention, the bridges consider the number ofnodes attached to the attached node as well as the logical distance ofthe attached node from the root node. If an attached node is overloadedwith other attached nodes, the unattached bridge may request attachmentto the less loaded node, or to a more loaded node as described above innetworks having regions of substantial RF overlap. In yet anotherembodiment, to avoid instability in the spanning tree, the bridge wouldonly conclude to change attachment if the logical distance of thepotential replacement is greater than a threshold value.

If no change in attachment is concluded, at a block 217 the bridgebranches back to the block 211. If a determination is made to changeattachment, a DETACH packet is sent to the root as illustrated at ablock 219. After sending the DETACH packet, the bridge branches back tothe block 205 to attach to the new spanning tree node. Note that theorder of shown for detachment and attachment is only illustrative andcan be reversed.

Referring back to the block 214, if the received packet (at block 211)is not a HELLO packet, the bridge branches to a block 221 to forward thereceived packet through the spanning tree. Afterwards, the bridgebranches back to the block 211 to continue the process.

Specifically, once attached, the attached bridge begins broadcastingHELLO packets (at the block 211) seeking to have all unattached bridges(or other network devices) attach to the attached bridge. Upon receivingan ATTACH.request packet, the bridge forwards that packet toward theroot node (through the blocks 211, 213, 214 and 221. On its path towardthe root, each node records the necessary information of how to reachrequesting bridge. This process is called “backward learning” herein,and is discussed more fully below. As a result of the backward learning,once the root node receives the ATTACH.request packet, anATTACH.response packet can be sent through the spanning tree to thebridge requesting attachment.

After attaching to an attached node, the newly attached bridge (thechild) must determine its distance from the root node. To arrive at thedistance of the child from the root node, the child adds the broadcastdistance of its parent from the root node to the distance of the childfrom its parent. In the preferred embodiment, the distance of a childfrom its parent is based on the bandwidth of the data communicationlink. For example, if the child attaches to its parent via a hard-wiredlink (data rate 26,000 baud), then the distance of that communicationlink might equal, for example, one hop. However, if the child attachesto its parent via an RF link (data rate 9600 baud), then the distance ofthat communication link might correspondingly be equal 3 hops. Thenumber of the hop corresponds directly to the communication speed of thelink. This may not only take into consideration baud rate, but also suchfactors as channel interference.

Initially, only the root gateway node 20 is broadcasting HELLO messagesand only nodes 40, 42 and 44 are within range of the HELLO messagesbroadcast by the gateway. Therefore, after the listening period hasexpired, nodes 40, 42 and 44 request attachment to the gateway node 20.The unattached nodes 40, 42, and 44 send ATTACH.request packets and theattached gateway node 20 acknowledges the ATTACH.request packets withlocal ATTACH.confirm packets. The newly attached bridges are assignedthe status ATTACHED and begin broadcasting their own HELLO packets,looking for other unattached bridges. Again, the remaining unattachednodes attempt to attach to the attached nodes that are logically closestto the root node. For example, node 48 is within range of HELLO messagesfrom both nodes 40 and 42. However, node 40 is three hops, via an RFlink, away from the gateway root node 20 and node 42 is only one hop,via a hard-wired link, away from the gateway root node 20. Therefore,node 48 attaches to node 42, the closest nods to the gateway root node20.

The sending of HELLO messages, ATTACH.request packets and ATTACH.confirmpackets continues until the entire spanning tree is established. Inaddition, attached bridges may also respond to HELLO messages. If aHELLO message indicates that a much closer route to the root node isavailable, the attached bridge sends a DETACH packet to its old parentand an ATTACH.request packet to the closer node. To avoid instability inthe system and to avoid overloading any given node, an attached bridgewould only respond to a HELLO message if the hop count in a HELLO packetis greater than a certain threshold value, CHANGE_THRESHOLD. In thepreferred embodiment, the value of the CHANGE_THRESHOLD equals 3. Inthis manner, an optimal spanning tree is formed that is capable oftransmitting data without looping.

Nodes, other than the gateway root node, after acknowledging anATTACH.request packet from a previously unattached node, will send theATTACH.request packet up the branches of the spanning tree to thegateway root node. As the ATTACH.request packet is being sent to thegateway root node, other nodes attached on the same branch record thedestination of the newly attached node in their routing entry table.When the ATTACH.request packet reaches the gateway root node, thegateway root node returns an end-to-end ATTACH.confirm packet.

After the spanning tree is initialized, the RF terminals listen forperiodically broadcasted Hello packets to determine which attached nodesare in range. After receiving HELLO messages from attached nodes, an RFterminal responding to an appropriate poll sends an ATTACH.requestpacket to attach to the node logically closest to the root. For example,RF terminal 110 is physically closer to node 44. However, node 44 isthree hops, via an RF link, away from the gateway root node 20 and node42 is only one hop, via a hard-wired link, away from the gateway rootnode 20. Therefore, RF terminal 110, after hearing HELLO messages fromboth nodes 42 and 44, attaches to node 42, the closest node to thegateway root node 20. Similarly, RF terminal 114 hears HELLO messagesfrom nodes 48 and 50. Nodes 48 and 50 are both four hops away from thegateway root node 20. However, node 48 has two RF terminals 110 and 112already attached to it while node 50 has only one RF terminal 116attached to it. Therefore, RF terminal 114 will attach to node 50, theleast busy node of equal distance to the gateway root node 20. Attachingto the least busy node proves to be the most efficient practice when thecommunication system has little overlap in the RF communication regions.In another embodiment, however, instead of attaching to the least busynode of equal distance to the gateway root node 20, the attachment isestablished with the busiest node.

The attached node acknowledges the ATTACH.request and sends theATTACH.request packet to the gateway root node. Then, the gateway rootnode returns an end-to-end ATTACH.confirm packet. In this manner, theend-to-end ATTACH.request functions as a discovery packet enabling thegateway root node, and all other nodes along the same branch, to learnthe address of the RF terminal quickly. This process is called backwardlearning. Nodes learn the addresses of terminals by monitoring thetraffic from terminals to the root. If a packet arrives from a terminalthat is not contained in the routing table of the node, an entry is madein the routing table. The entry includes the terminal address and theaddress of the node that sent the packet. In addition, an entry timer isset for that terminal. The entry timer is used to determine when RFterminals are actively using the attached node. Nodes maintain entriesonly for terminals that are actively using the node for communication.If the entry timer expires due to lack of communication, the RF terminalentry is purged from the routing table.

The RF links among the RF terminals, the bridges, and the gateway areoften lost. Therefore, a connection-oriented data-link service is usedto maintain the logical node-to-node links. In the absence of networktraffic, periodic messages are sent and received to ensure the stabilityof the RF link. As a result, the loss of a link is quickly detected andthe RF Network can attempt to establish a new RF link before datatransmission from the host computer to an RF terminal is adverselyaffected.

Communication between terminals and the host computer is accomplished byusing the resulting RF Network. To communicate with the host computer,an RF terminal sends a data packet in response to a poll from the bridgeclosest to the host computer.

Typically, the RF terminal is attached to the bridge closest to the hostcomputer. However, RF terminals are constantly listening for HELLO andpolling messages from other bridges and may attach to, and thencommunicate with, a bridge in the table of bridges that is closer to theparticular RF terminal.

Under certain operating conditions, duplicate data packets can betransmitted in the RF Network. For example, it is possible for an RFterminal to transmit a data packet to its attached node, for the node totransmit the acknowledgement frame, and for the RF terminal not toreceive the acknowledgement. Under such circumstances, the RF terminalwill retransmit the data. If the duplicate data packet is updated intothe database of the host computer, the database would become corrupt.Therefore, the RF Network of the present invention detects duplicatedata packets. To ensure data integrity, each set of data transmissionsreceives a sequence number. The sequence numbers are continuouslyincremented, and duplicate sequence numbers are not accepted.

When a bridge receives a data packet from a terminal directed to thehost computer, the bridge forwards the data packet to the parent node onthe branch. The parent node then forwards the data packet to its parentnode. The forwarding of the data packet continues until the gateway rootnode receives the data packet and sends it to the host computer.Similarly, when a packet arrives at a node from the host computerdirected to an RF terminal, the node checks its routing entry table andforwards the data packet to its child node which is along the branchdestined for the RF terminal. It is not necessary for the nodes alongthe branch containing the RF terminal to know the ultimate location ofthe RF terminal. The forwarding of the data packet continues until thedata packet reaches the final node on the branch, which then forwardsthe data packet directly to the terminal itself.

Communication is also possible between RF terminals. To communicate withanother RF terminal, the RF terminal sends a data packet to its attachedbridge. When the bridge receives the data packet from a terminaldirected to the host computer, the bridge checks to see if thedestination address of the RF terminal is located within its routingtable. If it is, the bridge simply sends the message to the intended RFterminal. It not, the bridge forwards the data packet to its parentnode. The forwarding of the data packet up the branch continues until acommon parent between the RF terminals is found. Then, the common parent(often the gateway node itself) sends the data packet to the intended RFterminal via the branches of the RF Network.

During the normal operation of the RF Network, RF terminals can becomelost or unattached to their attached node. If an RF terminal becomesunattached, for whatever reason, its routing entry is purged and the RFterminal listens for HELLO or polling messages from any attached nodesin range. After receiving HELLO or polling messages from attached nodes,the RF terminal sends an ATTACH.request packet to the attached nodeclosest to the root. That attached node acknowledges the ATTACH.requestand sends the ATTACH.request packet onto the gateway root node. Then,the gateway root node returns an end-to-end ATTACH.confirm packet.

Bridges can also become lost or unattached during normal operations ofthe RF Network. If a bridge becomes lost or unattached, all routingentries containing the bridge are purged. The bridge then broadcasts aHELLO.request with a global bridge destination address. Attached nodeswill broadcast HELLO packets immediately if they receive anATTACH.request packet with a global destination address. This helps thelost node re-attach. Then, the bridge enters the LISTEN state to learnwhich attached nodes are within range. The unattached bridge analyzesthe contents of broadcast HELLO messages to determine whether to requestattachment to the broadcasting node. Again, the bridge attempts toattach to the node that is logically closest to the root node. Afterattaching to the closest node, the bridge begins broadcasting HELLOmessages to solicit ATTACH.requests from other nodes or RF terminals.

The spread-spectrum system provides a hierarchical radio frequencynetwork of on-line terminals for data entry and message transfer in amobile environment. The network is characterized by sporadic datatraffic over multiple-hop data paths consisting of RS485 or ethernetwired links and single-channel direct sequenced spread spectrum links.The network architecture is complicated by moving, hidden, and sleepingnodes. The spread spectrum system consists of the following types ofdevices:

Terminal controller—A gateway which passes messages from a host port tothe RF network; and which passes messages from the network to the hostport. The host port (directly or indirectly) provides a link between thecontroller and a “host” computer to which the terminals are logicallyattached.

Base station—An intermediate relay node which is used to extend therange of the controller node. Base station-to-controller or basestation-to-base station-links can be wired or wireless RF.

Terminal—Norand RF hand-held terminals, printers, etc. In addition, acontroller device has a terminal component.

The devices are logically organized as nodes in an (optimal) spanningtree, with the controller at the root, internal nodes in base stationsor controllers on branches of the tree, and terminal nodes as (possiblymobile) leaves on the tree. Like a sink tree, nodes closer to the rootof the spanning tree are said to be “downstream” from nodes which arefurther away. Conversely, all nodes are “upstream” from the root.Packets are only sent along branches of the spanning tree. Nodes in thenetwork use a “BACKWARD LEARNING” technique to route packets along thebranches of the spanning tree.

Devices in the spanning tree are logically categorized as one of thefollowing three node types:

1) Root (or root bridge)—A controller device which functions as the rootbridge of the network spanning tree. In the preferred embodiment, thespanning tree has a single root node. Initially, all controllers areroot candidates from which a root node is selected. This selection maybe based on the hopping distance to the host, preset priority, randomselection, etc.

2) Bridge—An internal node in the spanning tree which is used to“bridge” terminal nodes together into an interconnected network. Theroot node is also considered a bridge and the term “bridge” may be usedto refer to all non-terminal nodes or all non-terminal nodes except theroot, depending on the context herein. A bridge node consists of anetwork interface function and a routing function.

3) Terminal—leaf node in the spanning tree. A terminal node can beviewed as the software entity that terminates a branch in the spanningtree.

A controller device contains a terminal node(s) and a bridge node. Thebridge node is the root node if the controller is functioning as theroot bridge. A base station contains a bridge node. A terminal devicecontains a terminal node and must have a network interface function. A“bridging entity” refers to a bridge node or to the network interfacefunction in a terminal.

The basic requirements of the system are the following.

a) Wired or wireless node connections.

b) Network layer transparency.

c) Dynamic/automatic network routing configuration.

d) Terminal mobility. Terminals should be able to move about the RFnetwork without losing an end-to-end connection.

e) Ability to accommodate sleeping terminals.

f) Ability to locate terminals quickly.

g) Built-in redundancy. Lost nodes should have minimal impact on thenetwork.

h) Physical link independence. The bridging algorithm is consistentacross heterogeneous physical links.

The software for the spread-spectrum system is functionally layered asfollows.

Medium Access Control (MAC)

The MAC layer is responsible for providing reliable transmission betweenany two nodes in the network (i.e. terminal-to-bridge). The MAC has achannel access control component and a link control component. The linkcontrol component facilitates and regulates point-to-point frametransfers in the absence of collision detection. The MAC channel accesscontrol component regulates access to the network. Note that herein, theMAC layer is also referred to as the Data Link layer.

Bridging Layer

The bridging layer, which is also referred to herein as the networklayer, has several functions as follows.

1. The bridging layer uses a “HELLO protocol” to organize nodes in thenetwork into an optimal spanning tree rooted at the root bridge. Thespanning tree is used to prevent loops in the topology. Interiorbranches of the spanning tree are relatively stable (i.e. controller andrelay stations do not move often). Terminals, which are leaves on thespanning three, may become unattached, and must be reattached,frequently.

2. The bridging layer routes packets from terminals to the host, fromthe host to terminals, and from terminals to terminals along branches ofthe spanning tree.

3. The bridging layer provides a service for storing packets forSLEEPING terminals. Packets which cannot be delivered immediately can besaved by the bridging entity in a parent node for one or more HELLOtimes.

4. The bridging layer propagates lost node information throughout thespanning tree.

5. The bridging layer maintains the spanning tree links.

6. The bridging layer distributes network interface addresses.

Logical Link Control Layer

A logical link control layer, also known herein as the Transport layerherein, is responsible for providing reliable transmission between anytwo nodes in the network (i.e., terminal-to-base station). The data-linklayer provides a connection-oriented reliable service and aconnectionless unreliable service. The reliable service detects anddiscards duplicate packets and retransmits lost packets. The unreliableservices provides a datagram facility for upper layer protocols whichprovide a reliable end-to-end data path. The data-link layer providesISO layer 2 services for terminal-to-host application sessions which runon top of an end-to-end terminal-to-host transport protocol. However,the data-link layer provides transport (ISO layer 4) services forsessions contained within the SST network.

Higher Layers

For terminal-to-terminal sessions contained within the SST network, thedata-link layer provides transport layer services and no additionalnetwork or transport layer is required. In this case, the MAC, bridging,and data-link layers discussed above can be viewed as a data-link layer,a network layer, and a transport layer, respectively. Forterminal-to-host-application sessions, higher ISO layers exist on top ofthe SST data-link layer and must be implemented in the terminal and hostcomputer, as required. This document does not define (or restrict) thoselayers. This document does discuss a fast-connect VMTP-like transportprotocol which is used for transient internal terminal-to-terminalsessions.

Specifically, a network layer has several functions, as follows.

1) The network layer uses a “hello protocol” to organize nodes in thenetwork into an optimal spanning tree rooted at the controller. (Aspanning tree is required to prevent loops in the topology.) Interiorbranches of the spanning tree are relatively stable (i.e., thecontroller and base stations do not move often). Terminals, which areleaves on the spanning tree, become unattached, and must be reattachedfrequently.

2) The network layer routes messages from terminals to the host, fromthe host to terminals, and from terminals to terminals along branches ofthe spanning tree.

3) The network layer provides a service for storing messages forSLEEPING terminals. Messages which cannot be delivered immediately canbe saved by the network entity in a parent node for one or more hellotimes.

4) The network layer propagates lost node information throughout thespanning tree.

5) The network layer maintains the spanning tree links in the absence ofregular data traffic.

A transport layer is responsible for establishing and maintaining areliable end-to-end data path between transport access points in any twonodes in the network. The transport layer provides unreliable, reliableand a transaction-oriented services. The transport layer should beimmune to implementation changes in the network layer.

The responsibilities of the transport layer include the following.

1) Establishing and maintaining TCP-like connections for reliableroot-to-terminal data transmission.

2) Maintaining VMTP-like transaction records for reliable transientmessage passing between any two nodes.

3) Detecting and discarding duplicate packets.

4) Retransmitting lost packets.

Layers 1 through 4 are self-contained within the Norand RF network, andare independent of the host computer and of terminal applications. Thesession layer (and any higher layers) are dependent on specificapplications. Therefore, the session protocol (and higher protocols)must be implemented as required. Note that a single transport accesspoint is sufficient to handle single sessions with multiple nodes.Multiple concurrent sessions between any two nodes could be handled witha session identifier in a session header.

Network address requirements are as follows. DLC framed contain a hopdestination and source address in the DLC header. Network packetscontain an end-to-end destination and a source address in the networkheader. Transport messages do not contain an address field; instead, atransport connection is defined by network layer source and destinationaddress pairs. Multiple transport connections require multiple networkaddress pairs.

The transport header contains a TRANSPORT ACCESS POINT identifier. DLCand network addresses are consistent and have the same format. Each nodehas a unique LONG ADDRESS which is programmed into the node at thefactory. The long address is used only to obtain a SHORT ADDRESS fromthe root node.

The network entity in each node obtains a SHORT ADDRESS from the rootnode, which identifies the node uniquely. The network entity passes theshort address to the DLC entity. Short addresses are used to minimizepacket sizes.

Short addresses consist of the following. There is: an address lengthbit (short or long).

a spanning tree identified.

a node-type identifier. Node types are well known.

a unique multi-cast or broadcast node identifier.

The node-identifier parts of root addresses are well known and areconstant. A default spanning tree identifier is well known by all nodes.A non-default spanning tree identifier can be entered into the root node(i.e., by a network administrator) and advertised to all other nodes in“hello” packets. The list of non-default spanning trees to which othernodes can attach must be entered into each node.

A node-type identifier of all 1's is used to specify all node types. Anode identifier of all 1's is used to specify all nodes of the specifiedtype. A DLC identifier of all 0's is used to specify a DLC entity whichdoes not yet have an address. The all-0's address is used in DLC framesthat are used to send, and receive network ADDRESS packets. (The networkentity in each node filters ADDRESS packets based on the networkaddress.)

Short-address allocation is accomplished as follows. Short nodeidentifiers of root nodes are well known. All other nodes must obtain ashort node identifier from the root. To obtain a short address, a nodesend an ADDRESS request packet to the root node. The source addresses(i.e., DLC and network) in the request packet are LONG ADDRESSES. Theroot maintains an address queue of used and unused SHORT ADDRESSES. Ifpossible, the root selects an available short address, associates theshort address with the long address of the requesting node, and returnsthe short address to the requesting node in an ADDRESS acknowledgepacket. (Note that the destination address in the acknowledge packet isa long address.)

A node must obtain a (new) short address initially and whenever anADDRESS-TIMEOUT inactivity period expires without having the nodereceive a packet from the network entity in the root.

The network entity in the root maintains addresses in the address queuein least recently used order. Whenever a packet is received, the sourceaddress is moved to the end of the queue. The address at the head of thequeue is available for use by a requesting node if it has never beenused or if it has been inactive for a MAX-ADDRESS-LIFE time period.

MAX-ADDRESS-LIFE must be larger than ADDRESS-TIMEOUT to ensure that anaddress is not in use by any node when it becomes available for anothernode. If the root receives an ADDRESS request from a source for which anentry exists in the address queue, the root simply updates the queue andreturns the old address.

The network layer organizes nodes into an optimal spanning tree with thecontroller at the root of the tree. (Note that the spanning threeidentifier allows two logical trees to exist in the same coverage area.)Spanning tree organization is facilitated with a HELLO protocol whichallows nodes to determine the shortest path to the root before attachingto the spanning tree. All messages are routed along branches of thespanning tree.

Nodes in the network are generally categorized as ATTACHED orUNATTACHED. Initially, only the root node is attached. A singlecontroller may be designated as the root, or multiple root candidates(i.e. controllers) may negotiate to determine which node is the root.Attached bridge nodes and root candidates transmit “HELLO” packets atcalculated intervals. The HELLO packets include:

a) the source address, which includes the spanning tree ID).

b) a broadcast destination address.

c) a “seed” value from which the time schedule of future hello messagescan be calculated.

d) a hello slot displacement time specifying an actual variation thatwill occur in the scheduled arrival of the very next hello message (thescheduled arrival being calculated from the “seed”).

e) the distance (i.e., path cost) of the transmitter from the host. Theincremental portion of the distance between a node and its parent isprimarily a function of the type of physical link (i.e., ethernet,RS485, RF, or the like). If a signal-strength indicator is available,connections are biased toward the link with the best signal strength.The distance component is intended to bias path selection toward (i.e.,wired) high-speed connections. Setting a minimum signal strengththreshold helps prevent sporadic changes in the network. In addition,connections can be biased to balance the load (i.e., the number ofchildren) on a parent node.

f) a pending message list. Pending message lists consist of 0 or moredestination-address/message-length pairs. Pending messages for terminalsare stored in the terminal's parent node.

g) a detached-node list. Detached-node lists contain the addresses ofnodes which have detached from the spanning tree. The root maintains twolists. A private list consists of all detached node addresses, and anadvertised list consists of the addresses of all detached nodes whichhave pending transport messages. The addresses in the hello packet areequivalent to the advertised list.

An internal node learns which entries should be in its list from hellomessages transmitted by its parent node. The root node builds itsdetached-node lists from information received in DETACH packets. Entriesare included in hello messages for DETACH-MSG-LIFE hello times.

Attached notes broadcast “SHORT HELLO” messages immediately if theyreceive an “HELLO.request” packet with a global destination address;otherwise, attached nodes will only broadcast hello messages atcalculated time intervals in “hello slots.” Short hello messages do notcontain a pending-message or detached-node list. Short hello messagesare sent independently of regular hello messages and do not affectregular hello timing.

Unattached nodes (nodes without a parent in the spanning tree) are,initially, in an “UNATTACHED LISTEN” state. During the listen state, anode learns which attached base station/controller is closest to theroot node by listening to hello messages. After the listening periodexpires an unattached node sends an ATTACH.request packet to theattached node closest to the root. The attached node immediatelyacknowledges the ATTACH.request, and send the ATTACH.request packet ontothe root (controller) node. The root node returns the request as anend-to-end ATTACH.confirm packet. If the newly-attached node is a basestation, the node calculates its link distance and adds the distance tothe distance of its parent before beginning to transmit hello messages.

The end-to-end ATTACH.request functions as a discovery packet, andenables the root node to learn the address of the source node quickly.The end-to-end ATTACH. request, when sent from a node to the root, doesnot always travel the entire distance. When a downstream node receivesan ATTACH.request packet and already has a correct routing entry for theassociated node, the downstream node intercepts the request and returnsthe ATTACH.confirm to the source node. (Note that any data piggy-backedon the ATTACH.request packet must still be forwarded to the host.) Thissituation occurs whenever a “new” path has more than one node in commonwith the “old” path.

The LISTEN state ends after MIN_HELLO hello time slots if hello messageshave been received from at least one node. If no hello messages havebeen received the listening node waits and retries later.

An attached node may respond to a hello message from a node other thanits parent (i.e., with an ATTACH.request) if the difference in the hopcount specified in the hello packet exceeds a CHANGE-THRESHOLD level.

Unattached nodes may broadcast a GLOBAL ATTACH.request with a multi-castbase station destination address to solicit short hello messages fromattached base stations. The net effect is that the LISTEN state may(optionally) be shortened. (Note that only attached base station or thecontroller may respond to ATTACH. requests.) Normally, this facility isreserved for base stations with children and terminals with transactionsin progress.

ATTACH. requests contain a (possibly empty) CHILD LIST, to enableinternal nodes to update their routing tables. ATTACH.requests alsocontain a “count” field which indicates that a terminal may be SLEEPING.The network entity in the parent of a SLEEPING terminal con temporarilystore messages for later delivery. If the count field is non-zero, thenetwork entity in a parent node will store pending messages until 1) themessage is delivered, or 2) “count” hello times have expired.

Transport layer data can be piggy-backed on an attached request packetfrom a terminal. (i.e., an attach request/confirm can be implementedwith a bit flag in the network header of a data packet.)

Network Layer Routing

All messages are routed along branches of the spanning tree. Basestations “learn” the address of terminals by monitoring traffic fromterminals (i.e., to the root). When a base station receives (i.e., anATTACH.request) packet, destined for the root, the base station createsor updates an entry in its routing table for the terminal. The entryincludes the terminal address, and the address of the base station whichsent the packet (i.e., the hop address). When a base station receives anupstream packet (i.e., from the root, destined for a terminal) thepacket is simply forwarded to the base station which is in the routingentry for the destination. Upstream messages (i.e., to a terminal) arediscarded whenever a routing entry does not exist. Downstream messages(i.e., from a terminal to the root) are simply forwarded to the nextdownstream node (i.e., the parent in the branch of the spanning tree.

TERMINAL-TO-TERMINAL COMMUNICATIONS is accomplished by routing allterminal-to-terminal traffic through the nearest common ancestor. In theworst case, the root is the nearest common ancestor. A “ADDRESS SERVER”facilitates terminal-to-terminal communications (see below).

DELETING INVALID ROUTING TABLE ENTRIES is accomplished in several ways:connection oriented transport layer ensures that packets will arrivefrom nodes attached to the branch of the spanning tree within thetimeout period, unless a node is disconnected.)

2) Whenever the DLC entity in a parent fails RETRY MAX times to send amessage to a child node, the node is logically disconnected from thespanning tree, with one exception. If the child is a SLEEPING terminal,the message is retained by the network entity in the parent for “count”hello times. The parent immediately attempts to deliver the messageafter it sends its next hello packet. If, after “count” hello times, themessage cannot be delivered, then the child is logically detached fromthe spanning tree. Detached node information is propagated downstream tothe root node, each node in the path of the DETACH packet must adjustits routing tables appropriately according to the following rules: a) ifthe lost node is a child terminal node, the routing entry for theterminal is deleted and a DETACH packet is generated, b) if the nodespecified in DETACH packet is a terminal and the node which deliveredthe packet is the next hop in the path to the terminal, then the routingtable entry for the terminal is deleted and the DETACH packet isforwarded, c) if the lost node is a child base station node then allrouting entries which specify that base station as the next hop aredeleted and a DETACH packet is generated for each lost terminal.

IN GENERAL, WHENEVER A NODE DISCOVERS THAT A TERMINAL IS DETACHED, ITPURGES ITS ROUTING ENTRY FOR THE TERMINAL. WHENEVER A NODE DISCOVERSTHAT A BASE STATION IS DETACHED, IT PURGES ALL ROUTING ENTRIESCONTAINING THE BASE STATION. ONLY. ENTRIES FOR UPSTREAM NODES AREDELETED.

When DETACH packets reach the root node, they are added to a “detachedlist.” Nodes remain in the root node's detached list until a) the nodereattaches to the spanning tree, or b) the list entry times out. Thedetached list is included in hello messages and is propagated throughoutthe spanning tree.

For example, if a terminal detaches and reattaches to a different branchin the spanning tree, all downstream nodes in the new branch (quickly)“learn” the new path to the terminal. Nodes which were also in the oldpath change their routing tables and no longer forward packets along theold path. At least one node, the root, must-be in both the old and newpath. A new path is established as soon as an end-to-end attach requestpacket from the terminal reaches a node which was also in the old path.

4) A node (quickly) learns that it is detached whenever it receives ahello message, from any node, with its address in the associateddetached list. The detached node can, optionally, send a globalATTACH.request, and then enters the UNATTACHED LISTEN state andreattaches as described above. After reattaching, the node must remainin a HOLD-DOWN state until its address is aged out of all detachedlists. During the HOLD-DOWN state the node ignores detached lists.

5) A node becomes disconnected and enters the UNATTACHED LISTEN statewhenever HELLO-RETRY-MAX hello messages are missed from its parent node.

6) A node enters the ATTACHED LISTEN state whenever a single hellomessage, from its parent, is missed. SLEEPING terminals remain awakeduring the ATTACHED LISTEN state. The state ends when the terminalreceives a data or hello message from its parent. The terminal becomesUNATTACHED when a) its address appears in the detached list of a hellomessage from an ode other than its parent, or b) HELLO-RETRY-MAX hellomessages are missed. The total number of hello slots spend in the LISTENstate is constant.

If a node in the ATTACHED LISTEN state discovers a path to the rootwhich is CHANGE-THRESHOLD shorter, it can attach to the shorter path.Periodically, SLEEPING terminals must enter the ATTACHED LEARN state todiscovery any changes (i.e., shorter paths) in the network topology.

Hello Synchronization

All attached non-terminal nodes broadcast periodic “hello” messages indiscrete “hello slots” at calculated intervals. Base station nodes learnwhich hello slots are busy and refrain from transmitting during busyhello slots.

A terminal refrains from transmitting during the hello slot of itsparent node and refrains from transmitting during message slots reservedin a hello message.

The hello message contains a “seed” field used in a well-knownrandomization algorithm to determine the next hello slot for thetransmitting node and the next seed. The address of the transmittingnode is used as a factor in the algorithm to guarantee randomization.Nodes can execute the algorithm i times to determine the time (and seed)if the i-the hello message from the transmitter.

After attached, a base station chooses a random initial seed and anon-busy hello slot and broadcasts a hello message in that slot. Thebase station chooses succeeding hello slots by executing therandomization algorithm. If an execution of the algorithm chooses a busyslot, the next free slot is used and a hello “displacement” fieldindicates the offset from a calculated slot. Cumulative delays are notallowed (i.e., contention delays during the i hello transmission do noteffect the time of the i+1 hello transmission).

HELLO-TIME and HELLO-SLOT-TIME values are set by the root node andflooded throughout the network in hello messages. The HELLO-SLOT-TIMEvalue must be large enough to minimize hello contention.

A node initially synchronizes on a hello message from its parent. ASLEEPING node can power-down with an active timer interrupt to wake itjust before the next expected hello message. The network entity in basestation nodes can store messages for SLEEPING nodes and transmit themimmediately following the hello messages. This implementation enablesSLEEPING terminals to receive unsolicited messages. (Note that thenetwork layer always tries to deliver messages immediately, beforestoring them.) Retries for pending messages are transmitted in around-robin order when messages are pending for more than onedestination.

Note that a child node that misses i hello messages, can calculate thetime of the i+1 hello message.

Transport Layer Theory and Implementation Notes

The transport layer provides reliable, unreliable, andtransaction-oriented services. Two types of transport connections aredefined: 1) a TCP-like transport connection may be explicitly requestedfor long-lived connections or 2) a VMTP-like connection-record may beimplicitly set up for transient connections. In addition, aconnectionless service is provided for nodes which support an end-to-endtransport connection with the host computer.

The interfaces to the next upper (i.e., application) layer include:

CONNECT (access_point, node_name)

LISTEN (access_point)

UNITDATA (access_point, node_name, buffer, length)

SEND (handle, buffer, length)

RECEIVE (handle, buffer, length)

CLOSE (handle)

The “handle” designates the connection type, and is the connectionidentifier for TCP-like connections.

SEND messages require a response from the network node (root orterminal) to which the message is directed.

UNITDATA messages do not require a response. UNITDATA is used to sendmessages to a host which is capable of supporting end-to-endhost-to-terminal transport connections.

Because the network layer provides an unreliable service, the transportlayer is required to detect duplicate packets and retransmit lostpackets. Detecting duplicates is facilitated by numbering transportpackets with unambiguous sequence numbers.

Transport Connections

TCP-like transport connections are used for message transmission overlong-lived connections. The connections may be terminal-to-root orterminal-to-terminal (i.e., base stations are not involved in thetransport connection).

TCP-like transport connections are established using a 3-way handshake.Each end selects its initial sequence number and acknowledges the otherend's initial sequence number during the handshake. The node whichinitiates the connection must wait a MAX-PACKET-LIFE time, beforerequesting a connection, to guarantee that initial sequence numbers areunambiguous. Sequence numbers are incremented modulo MAX-SEQ, whereMAX-SEQ is large enough to insure that duplicate sequence numbers do notexist in the network. Packet types for establishing and breakingconnections are defined as in TCP.

A TCP-like connection is full-duplex and a sliding window is used toallow multiple outstanding transport packets. An ARQ bit in thetransport header is used to require an immediate acknowledgment from theopposite end.

VMTP-like connections are used for transient messages (i.e.terminal-to-terminal mail messages). VMTP-like connection records arebuilt automatically. A VMTP-like connection record is built (or updated)whenever a VMTP-like transport message is received. The advantage isthat an-explicit connection request is not required. The disadvantage isthat longer and more carefully selected sequence numbers are required. AVMTP-like connection is half-duplex. (A full-duplex connection at ahigher layer can be built with two independent half-duplex VMTP-likeconnections.) Acknowledgments must be handled by higher layers.

Transport connections are defined by the network end-to-end destinationand source addresses.

A MAX_TP_LIFE timeout is associated with transport connections.Transport connection records are purged after a MAX_TP_LIFE time expireswithout activity on the connection. The transport entity in a terminalcan ensure that its transport connection will not be lost bytransmitting an empty time-fill transport packet whenever TP_TIMEOUTtime expires without activity.

The transport entity in a node stores messages for possibleretransmission. Note that retransmissions may not always follow the samepath (primarily) due to moving terminals and the resulting changes inthe spanning tree. For example, the network entity in a parent node maydisconnect a child after the DLC entity reports a message deliveryfailure. The child will soon discover that it is detached and willreattach to the spanning tree. Now when the transport entity (i.e. inthe root) re-sends the message, it will follow the new path.

Transport Message Timing and Sleeping Terminals

The transport entity in a terminal calculates a separate timeout forSEND and TRANSACTION operations. Initially, both timeouts are a functionof the distance of the terminal from the root node.

A TCP-like algorithm is used to estimate the expected propagation delayfor each message type. Messages, which require a response, areretransmitted if twice the expected propagation time expires before aresponse is received. SLEEPING terminals can power down for a largepercentage of the expected propagation delay before waking up to receivethe response message. Note that missed messages may be stored by thenetwork layer for “count” hello times.

Medium Access Control (MAC) Theory and Implementation Notes

Access to the network communications channel is regulated in severalways: executing the full CSMA algorithm (see MAC layer above). Thesender retransmits unacknowledged messages until a RETRY_MAX count isexhausted.

The retry time of the DLC must be relatively short so that lost nodescan be detected quickly. When the DLC layer reports a failure to delivera message to the network layer, the network layer can 1) save messagesfor SLEEPING terminals for later attempts, or 2) DETACH the node fromthe spanning tree. Note that most lost nodes are due to movingterminals.

The node identifier part of the DLC address is initially all 0's for allnodes except the root node. The all 0's address is used by a node tosend and received data-link frames until a unique node identifier ispassed to the DLC entity in the node. (The unique node identifier isobtained by the network entity.)

Address Resolution

Well-known names too are bound to network addresses in several ways:

The network address and TRANSPORT ACCESS ID of a name server, containedin the root, is well-known by all nodes.

A node can register a well-known name with the name server contained inthe root node.

A node can request the network access address of another applicationfrom the name server by using the well-known name of the application.

Possible Extensions

Base station-to-base station traffic could also be routed through thecontroller if the backward learning algorithm included base stationnodes. (Each base station would simply have to remember which directionon its branch of the spanning tree to send data directed toward anotherbase station.)

The possibility of multiple controllers is kept open by including aspanning-tree identifier in address fields. Each controller defines aunique spanning tree. A node can be in more than one spanning tree, withseparate network state variables defined for each.

Thus, the preferred embodiment of the present invention describes anapparatus and a method of efficiently routing data through a network ofintermediate base stations in a radio data communication system.

In alternate embodiments of the present invention, the RF Networkscontain multiple gateways. By including a system identifier in theaddress field of the nodes, it is possible to determine which nodes areconnected to which networks.

Multipath Fading

In a preferred embodiment, the data to be sent through the RFcommunication link is segmented into a plurality of DATA packets and isthen transmitted. Upon receipt, the DATA packets are reassembled for useor storage. Data segmentation of the RF link provides bettercommunication channel efficiency by reducing the amount of data loss inthe network. For example, because collisions between transmissions on anRF link cannot be completely avoided, sending the data in small segmentsresults in an overall decrease in data loss in the network, i.e., onlythe small segments which collide have to be re-sent.

Similarly, choosing smaller data packets for transmission also reducesthe amount of data loss by reducing the inherent effects ofperturbations and fluctuations found in RF communication links. Inparticular, RF signals are inherently subject to what is termed“multi-path fading.” A signal received by a receiver is a composite ofall signals that have reached that receiver by taking all availablepaths from the transmitter. The received signal is therefore oftenreferred to as a “composite signal” which has a power envelope equal tothe vector sum of the individual components of the multi-path signalsreceived. If the signals making up the composite signal are ofamplitudes that add “out of phase”, the desired data signal decreases inamplitude. If the signal amplitudes are approximately equal, aneffective null (no detectable signal at the receiver) results. Thiscondition is termed “fading”.

Normally changes in the propagation environment occur relatively slowly,i.e., over periods of time ranging from several tenths ({fraction(1/10)}'s) of seconds to several seconds. However, in a mobile RFenvironment, receivers (or the corresponding transmitters) often travelover some distance in the course of receiving a message. Because thesignal energy at each receiver is determined by the paths that thesignal components take to reach that receiver, the relative motionbetween the receiver and the transmitter causes the receiver toexperience rapid fluctuations in signal energy. Such rapid fluctuationscan cause fading and result in the loss of data if the amplitude of thereceived signal falls below the sensitivity of the receiver.

Over small distances, the signal components that determine the compositesignal are well correlated, i.e., there is a small probability that asignificant change in the signal power envelope will occur over thedistance. If a transmission of a data packet can be initiated andcompleted before the relative movement between the receiver andtransmitter exceeds the “small distance”, data loss to fading isunlikely to occur. The maximum “small distance” wherein a high degree ofcorrelation exists is referred to hereafter as the “correlationdistance”.

As expressed in wavelengths of the carrier frequency, the correlationdistance is one half (½) of the wavelength, while a more conservativevalue is one quarter (¼) of the wavelength. Taking this correlationdistance into consideration, the size of the data packet forsegmentation purposes can be calculated. For example, at 915 MHz (apreferred RF transmission frequency), a quarter wavelength is about 8.2centimeters. A mobile radio moving at ten (10) miles per hour, or 447centimeters per second, travels the quarter wavelength in about 18.3milliseconds. In such an environment, as long as the segment packet sizeremains under 18.3 milliseconds, fading does not pose any problems. In apreferred embodiment, five (5) millisecond data packet segments arechosen which provides a quasi-static multipath communicationenvironment.

Duty Cycle

In a preferred embodiment, each base station broadcasts HELLO messagesabout every two (2) seconds. If upon power up, two base stations chooseto broadcast at the exact same broadcast, collisions between HELLOmessages would occur and continue to occur in a lock-step fashion uponeach broadcast. To prevent such an occurrence, each base station choosesa pseudo-random offset from the 2 second base time between HELLOmessages to actually broadcast the HELLO message. For example, insteadof beginning each HELLO message broadcast at exactly 2 seconds after thelast, the base station might pseudo-randomly offset the 2 seconds by anegative (−) value of 0.2, yielding a broadcast at 1.8 seconds. Becauseevery base station generates a different pseudo-random offsetgeneration, the problem of lock-stepping collisions is avoided.

Additionally, instead of using a true randomization, a pseudo-randomoffset is used which bases all pseudo-random offset calculations on aseed value (the “seed”). The “seed” is broadcast in each HELLO messageso that the timing of the next HELLO message may be calculated by anylistening mobile terminal. The use of the seed, and pseudo random offsetgeneration, allows the terminal to “sleep” (enter an energy and CPUsaving mode) between HELLO messages and be able to “wake up” (dedicateenergy and CPU concentration on RF reception) and stay awake for theminimal time needed to receive the next HELLO message. The relationshipbetween the time that a base station must remain awake to the time itmay sleep is called the “duty cycle”.

Using a 2 second HELLO to HELLO message timing with a pseudo-randomoffset range of +/− ⅓ of a second, the preferred embodiment has achieveda very low duty cycle. Further details of this timing can be found inthe Bridge Layer Specification in Appendix E.

In addition, Appendix A provides a list of the program modules which arefound in microfiche Appendix B. These modules comprise an exemplarycomputer program listing of the source code (“C” programming language)used by the network controllers and intelligent base transceivers of thepresent invention. Note that the term “AMX” found in Appendices A and Brefers to the operating system software used. “AMX” is a multi-taskingoperating system from KADAK Products, Ltd., Vancouver, B.C., Canada.Appendix C, D, E, F, and G provide system specifications for the SSTNetwork Architecture, SST Network Frame Format, Bridging Layer, MACLayer, and Physical Layer of one embodiment of the present invention.

As is evident from the description that is provided above, theimplementation of the present invention can vary greatly depending uponthe desired goal of the user. However, the scope of the presentinvention is intended to cover all variations and substitutions whichare and which may become apparent from the illustrative embodiment ofthe present invention that is provided above, and the scope of theinvention should be extended to the claimed invention and itsequivalents.

What is claimed is:
 1. A communication network supporting wirelesscommunication of messages, said communication network comprising: afirst terminal node having a wireless receiver operable in a normalstate; a second terminal node having a wireless receiver operable in apower saving state; an access point that attempts to immediately delivermessages destined for the first terminal node; the access point attemptsto deliver messages destined for the second terminal node bytransmitting at predetermined intervals beacons that identify that amessage awaits delivery; the second terminal node synchronizes operationof its wireless receiver to receive the beacons from the access point;and the second terminal node determines from the received beacons thatit has a message awaiting delivery and directs further operation of itswireless receiver to receive the message.
 2. The communication networkof claim 1 wherein the first terminal node selectively operates in oneof the normal mode and a power saving state and while operating in thepower saving state the first terminal node synchronizes operation of itswireless receiver to receive the beacons from the access point.
 3. Thecommunication network of claim 1 wherein the second terminal nodedirects further operation of its receiver to receive the message duringa time period that follows one of the received beacons.
 4. Thecommunication network of claim 3 wherein the time period immediatelyfollows the one of the received beacons.
 5. The communication network ofclaim 3 wherein the time period follows the one of the received beaconsduring an awake time window.
 6. The communication network of claim 5wherein the awake time window occurs an offset time following the one ofthe received beacons.
 7. The communication network of claim 3 whereinthe second terminal node has a wireless transmitter that is used torequest the message awaiting delivery.
 8. The communication network ofclaim 5 wherein the second terminal node has a wireless transmitter thatis used to request that the message awaiting delivery be deliveredduring the awake time window.
 9. The communication network of claim 6wherein the second terminal node has a wireless transmitter that is usedto request that the message awaiting delivery be delivered during theawake time window at the offset time, wherein the awake time window andthe offset time are communicated with the request.
 10. The communicationnetwork of claim 1 wherein the second terminal node communicates to theaccess point an indication of whether the second terminal node operatesin the power saving state.
 11. The communication network of claim 3wherein the access point queues the messages awaiting delivery, andremoves from the queue those of the messages awaiting delivery wheredelivery is unsuccessful.
 12. The communication network of claim 11wherein the messages awaiting delivery remain in the queue untildelivery is successful or until a predetermined number of the beaconsoccur wherein delivery is unsuccessful.
 13. The communication network ofclaim 3 wherein the second terminal node synchronizes operation of itswireless receiver to receive the beacons from the access point even whenone or more of the beacons from the access point have not been received.14. The communication network of claim 1 wherein the second terminalnode comprises a battery-powered, roaming device.
 15. The communicationnetwork of claim 14 wherein the access point participates in spanningtree routing to support the battery-powered, roaming device.
 16. Acommunication network supporting wireless communication of messages,said communication network comprising: a first terminal node operatingin a first state; a second terminal node operating in a second state inwhich attempts are made to minimize power consumption by the wirelessreceiver a bridging node having a wireless transceiver to supportwireless communication to the first and second terminal nodes; thebridging node attempts to deliver messages destined for the secondterminal node by transmitting at predetermined intervals beacons thatidentify a message awaiting delivery; the second terminal nodesynchronizing operation of its wireless receiver to receive the beaconsfrom the bridging node and determining from the received beacons that ithas a message awaiting delivery and responding to an awaiting message bydirecting further operation of its wireless receiver to receive themessage; and the bridging node delivering messages to the first terminalnode without requiring the first terminal node to determine from thebeacons that it has messages awaiting delivery.
 17. The communicationnetwork of claim 16 wherein the second terminal node directs furtheroperation of its receiver to receive the message during a time periodthat follows one of the received beacons.
 18. The communication networkof claim 17 wherein the time period immediately follows the one of thereceived beacons.
 19. The communication network of claim 17 wherein thetime period follows the one of the received beacons during an awake timewindow.
 20. A communication network supporting wireless communication ofmessages, said communication network comprising: a first node having awireless transceiver; a second node having a wireless transceiver; saidfirst node wirelessly transmitting at periodic intervals to accommodatedelivery of messages from said first node to said second node; and saidsecond node selectively either entering and remaining in a low powerstate between the transmissions at periodic intervals or entering andremaining in a low power state between any two of the transmissions atperiodic intervals that are nonconsecutive.
 21. The communicationnetwork of claim 20 wherein at least one of the first node and thesecond node comprising a roaming terminal.
 22. The communication networkof claim 21 wherein the second node directs further operation of itstransceiver to receive messages during a time period that follows one ofthe wireless transmissions from the first node.
 23. The communicationnetwork of claim 22 wherein the time period immediately follows the oneof the wireless transmissions from the first node.
 24. The communicationnetwork of claim 22 wherein the time period follows the one of thewireless transmissions from the first node during an awake time window.25. The communication network of claim 24 wherein the awake time windowoccurs an offset time following the one of the wireless transmissionsfrom the first node.
 26. A communication network supporting wirelesscommunication of messages, said communication network comprising: afirst node having a wireless transceiver; a second node having awireless receiver; said first node wirelessly transmitting at timedintervals to accommodate delivery of messages from said first node tosaid second node; and said second node synchronizing with the timedintervals to selectively enter and remain in a low power state eitherone of between consecutive transmissions at periodic intervals andbetween nonconsecutive transmissions at periodic intervals.
 27. Thecommunication network of claim 26 wherein at least one of the first nodeand the second node comprising a roaming terminal.
 28. The communicationnetwork of claim 27 wherein the second node directs further operation ofits transceiver to receive messages during a time period that followsone of the wireless transmissions from the first node.
 29. Thecommunication network of claim 28 wherein the time period immediatelyfollows the one of the wireless transmissions from the first node. 30.The communication network of claim 28 wherein the time period followsthe one of the wireless transmissions from the first node during anawake time window.
 31. The communication network of claim 30 wherein theawake time window occurs an offset time following the one of thewireless transmissions from the first node.